2 tablespoons olive oil
1 large onion, cut into rings
1 (15 ounce) can stewed tomatoes
1 (7 ounce) can chipotle peppers in adobo sauce, or to taste
2 pounds shredded cooked chicken meat
16 tostada shells
1/2 cup sour cream
Heat olive oil in a saucepan over medium heat. Add the onions; cook and stir until softened and translucent, about 5 minutes. Meanwhile, puree the tomatoes with chipotle peppers and adobo sauce to taste. Pour into the onions, and add chicken. Cover, and simmer for 20 minutes.
To serve, mound the chicken onto tostada shells, and garnish with a dollop of sour cream.